1. Field of the Invention
This invention relates to servo operated digital positioning control systems, and more particularly, to digital positioning control systems of the type stated which is capable of accurately positioning a dynamically moveable member in response to a position command signal on a highly efficient basis, and which can be used inexpensively in industrial and commercial applications.
2. Brief Description of the Prior Art
High accuracy positioning systems have been used for many years in a variety of military and aerospace applications to position moveable control members. Such applications include the positioning of antennas and optical sensors. A key element in any positioning system is the position transducer used to determine the position of the moveable control element since the overall performance of the positioning system is a function of the accuracy, repeatability and linearity of the position transducer. Historically, many types of transducers have been employed to measure position such as potentiometers, resolvers, differential transformers, magnetic encoders, optical encoders and the like.
Optical shaft encoders have found widespread use as position transducers in high accuracy positioning systems. In particular, absolute position optical encoders are employed to obtain the high resolution necessary for accurate positioning. These types of encoders include a light source such as a light emitting diode, a group of photo-sensors such as photo-diodes, and a coded disk positioned between the light source and the photo-sensors. If the control element to be positioned is the shaft of a motor, the coded disk is rotatably mounted to the motor shaft. The coded disk includes patterns of clear and opaque segments which vary in size and location according to the resolution required by the application. The patterns are arranged so that light passes from the light source through the clear segments of the disk and illuminates the photo-sensors in a prescribed manner whereby the photo sensor output signals form a binary representation of the absolute position of the motor shaft.
Further, in any positioning system and, particularly servo control positioning systems, a high quality motor, such as a servo type motor, is used for operation. It is desirable to position the moveable member to the command position as quickly as possible without overshoot, or undershoot, or other type of positioning error. For example, in the case of a servo system for moving a reading head in a computer, it is desirable to position the reading head at the proper track and sector, upon receiving the address or command position, in the most efficient and fastest manner available within the constraints of the system itself.
In many prior art servo systems, there are repeatable error components. For the most part, it is possible to compensate for these repeatable error components. However, it is difficult to compensate for the non-repeatable error components which may exist in the servo motor itself and elsewhere. After a period of time, it has been found that many other errors creep into most digital servo control systems and which do affect velocity and position factors.
In most positioning systems, the movement of the moveable member is constrained by acceleration factors, maximum possible velocity and the like. Thus, if a moveable member were to be moved from an initial starting position to a command position, it would be desirable to accelerate to a mid point between these two positions and thereafter decelerate to the command position. In most cases, this is not accurately achievable. In addition, it would be desirable to increase the velocity from the initial starting position to a maximum velocity and move at the member at that maximum velocity to a point in advance of the command position where deceleration starts with a velocity profile decrease so that the moveable member stops exactly at the command position. Here again, this is not effectively accurately obtained in most prior art systems. Failure to obtain optimum acceleration and velocity profiles is due in part to the fact that acceleration and velocity control is limited by the servo motor or other form of drive motor and various mechanical components in the system.
There have been many elaborate prior art controls which have been utilized in digital servo systems and which utilize some form of profile for obtaining proper positioning of a moveable member. For example, U.S. Pat. No. 3,881,184 to Koepcke et al discloses the use of an adaptive digital servo system having a track position profile mechanism for generating a sequence of binary position signals having a correction signal for each position signal in the position profile.
U.S. Pat. No. 3,994,016 to Moghadam also discloses a closed loop and head positioning servo system in which a track counting course positioning system is used along with a fine positioning system. U.S. Pat. No. 4,268,785 to Svendsen also discloses a system for compensating for transient overshoot and undershoot in the movement of a member in a position feedback system. U.S. Pat. No. 4,122,503 to Allan also discloses a positioning system used with floppy disks and which uses offset measurement and compensates for position error.
Accordingly, it is an object of the present invention to provide a new and improved digital positioning control system in which a moveable member can be moved to a command position at a relatively high speed with compensation for position error and velocity error in response to a position command signal.
It is another object of the present invention to provide a digital positioning system which utilizes a velocity profile for comparing a velocity command to a determined velocity.
It is a further object of the present invention to provide a digital positioning control system which is capable of generating highly accurate digital position and velocity control signals in response to the output signals from an incremental position optical encoder.
It is still another object of the present invention to provide a method of positioning a moveable member from an initial starting position to a command position by causing movement of the moveable member to a maximum velocity with movement at the maximum velocity until a position in advance of the command position at which point deceleration and concomitant velocity decrease are achieved until the moveable member reaches the command position.
With the above and other objects in view, my invention resides in the novel features of form, construction, arrangement and combination of components and parts presently described and pointed out.